Radial Engineering lists the differences on the FAQ section for both products. Their claim is that the active X-Amp has better 'reach' (extended HF response), and that the active design is less 'finnicky' about impedance loads, rendering more stable level and tone. There is also an
LED on the X-amp to help set levels.
FWIW, I own a ProRMP, and bought it because it DOES use an isolation transformer. Because I'm using it to...re-amp (hello!!!) sources, I don't usually want full-range 'hi-fi' response. A little bit of HF loss is no big deal, in fact, I've found it desireable. Less 'harshness' in the top end, if you will.
I've been experimenting reamping guitars, trumpets, synths, and bass, and have had no issues with it. It's been a great $100 tool.

All the Radial stuff is top notch. I've been looking into getting the Little Labs Redeye for my toolbox. A little more money, but it does the reamp thing AND can be used as a regular DI. I like the kill two birds with one stone approach.

Thanks for the help, you guys.